PHOTOCHEMISTRY OF N-PHTHALOYL DERIVATIVES OF ELECTRON-DONOR-SUBSTITUTED AMINO-ACIDS

The hydroxy substituted amino acids threonine and serine have been investigated concerning their photochemical behaviour when activated as N-phthaloyl substrates. The methyl esters la and 2a solely underwent cleavage of the central C-C single bond to give the glycine derivative 3 and an aldehyde fragment. C-unprotected threonine derivative Ib is converted into a series of products the composition of which depends on solvent polarity and on the electronic state. Three reaction modes were detected for the N-phthaloyl derivatives of the aryl substituted amino acids phenylalanine, tyrosine, and dihydroxyphenylalanine (DOPA): (A) decarboxylation (only for 12a), (B) beta-fragmentation, and (C) ring enlargement reaction. Processes B and C are initiated by photo electron transfer (PET), as the solvent dependence revealed. The DOPA derivatives 14a and 14b are the most prominent examples due to their exclusive PET reactivity leading to type C products(18a,b) with high diastereoselectivity (90:10). PET results from the first excited singlet states of 12 and 13, whereas for compounds 14 the corresponding triplet states are involved. The correlation between photochemical reactivity and the fluorescence decay data for compounds 12a,b and 15 is discussed.